This invention relates to an improved oxygen electrode for fuel cells utilizing an acid electrolyte and to a method for preparing the same.
Fuel cells are basically galvanic energy conversion devices which circumvent the limitations of the Carnot cycle. Conversion efficiency, therefore, can be high as compared with other conversion methods. Other considerations favor fuel cells in addition to high conversion efficiency. Since they are primarily low temperature conversion devices, they are largely without polluting emissions. Fuel cells are versatile with respect to size and power level which is partly reflected by an adaptability to modular design. Also important, fuel cells require few moving parts and therefore promise to be quiet, reliable and comparatively maintance-free.
As energy conversion devices, fuel cells are distinguished from conventional batteries by the fact that the electrodes are invariable and catalytically active. Current is generated by reaction on the electrode surfaces which are in contact with a suitable electrolyte and fuel or oxidant. As a rule, fuel and oxidant are not an integral part of the cell, but are supplied as required by the current load, and reaction products are continuously removed.
The nature of the fuel is a critical aspect of cell operation. Hydrogen combustion with air or oxygen is by far the most important reaction for power generation in a fuel cell, although fuels such as hydrazine and oxidants such as hydrogen peroxide are under consideration for specialized purposes.
For example, a typical hydrogen-air or oxygen cell consists of a container divided in two by a porous separator and filled with an acid electrolyte each half containing an electrode. The electrode reactions are comprised of the oxidation of hydrogen on the anode or negative electrode to hydrated protons with the release of electrons; and on the cathode the reaction of oxygen with protons to form water vapor with the consumption of electrons. Electrons flow from the anode through the external load to the cathode and the circuit is closed by ionic current transport through the electrolyte.
The electrode, in its catalyzed layer must provide a number of sites where gases and electrolyte can react. The electrode must also provide a path for current to flow to the terminals and in some cell designs may serve to contain the electrolyte.
Electrodes, particularly those for use in an acid electrolyte such as sulfuric or phorphoric acid, often times are constructed of platinum or of a layer of platinum on a electrically conductive substrate, such as carbon. However, electrodes such as these still require the use of a substantial quantity of expensive platinum metal to provide a sufficient number of catalytic sites in order to obtain adequate cell activity. Furthermore, the catalytic reaction is oftentimes destructive of the electrodes, particularly at the cathode or oxygen electrode.